Process and apparatus for the combustion of carbonaceous material



United States Patent [72] Inventors James McLaren and Derek F. Williams,Cheltenham, England 211 App]. No. 764,648

[54] PROCESS AND APPARATUS FOR THE COMBUSTION 0F CARBONACEOUS MATERIAL 3Claims, No Drawings U.S. Cl. 110/1 F23b 7/00 Field ofSearch llO/ll, U,

[56] References Cited UNITED STATES PATENTS 3,320,906 5/1967 Domahidy I10/1 FOREIGN PATENTS 768,656 2/1957 Great Britain I 10/] PrimaryExaminer- Kenneth W. Sprague AttorneyStevens, Davis, Miller & MosherABSTRACT: A process for the combustion of carbonaceous material such ascoal to reduce the sulphur content of the offgases involves burning thematerial in a fluidised bed. The fluidised bed comprises ash having lessthan 0.5 percent carbon and is held at a temperature of between 700 and900C. Carbonate such as calcium carbonate is added in proportion of 2-l4percent by weight to the material so that the sulphur isretained in thebed.

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PROCESS AND APPARATUS FOR THE COMBUSTION F CARBONACEOUS MATERIAL Thisinvention relates to a process and apparatus for the combustion ofcarbonaceous material within a fluidised bed.

In the combustion of carbonaceous material containing sulphur, the gasesof combustion normally carry away a major proportion of this sulphurthereby resulting in pollution of the atmosphere.

An object of the invention is to reduce the amount of sulphur dischargedwith the combustion gases during combustion of the carbonaceousmaterial.

For this purpose the invention provides a process for the combustion ofcarbonaceous material wherein the material is fed into a fluidised bedof ash containing not more than 0.5 percent by weight of carbon,carbonate such as calcium carbonate is also fed to the fluidised bed andthe carbonaceous material is burned within said bed at a temperature inthe range 700900C. Advantageously the feed of carbonate as a proportionof the carbonaceous material fed to the fluidised bed is between 2 14percent by weight.

By the addition of carbonate to the fluidised bed sulphur is morereadily retained in the ash and tines, and is not therefore carried outin the exhaust gases.

A proportion of the hot ash is continuously discharged from thefluidised bed according to the rate of feed of the incoming carbonaceousmaterial and heat transfer is effected between the fluidised bed andheat exchange medium circulating through the bed.

Conveniently a bed of ash is contained in a reactor which is fluidisedby a stream of upwardly flowing gas entering through openings in thebase of the reactor.

'Ash may be continuously removed from a discharge aper ture locatedtowards a lower part of the reactor and carbonaceous material may be fedby pneumatic means into the fluidised bed through an inlet aperture at arelatively upper level in the reactor.

Heat exchange medium may be circulated through tubes located within thefluidised bed. The hot medium in liquid or gaseous form may then be ledto a boiler or the like for electric power generation.

The carbonaceous material containing sulphur may be coal ground to asuitable fine size e.g., 0 10 8.8.8.

In addition to or as an alternative to calcium carbonate or limestone,other carbonate containing substance, for example dolomite, may be addedto the fluidised bed. Ferrous oxide may also be added to the bed.

In an example of the invention various coals were burnt in al fluidisedbed of ash 0.6 m. deep at 800C, the velocity of the} fluidising gasbeing 0.6 m/sec., and fines carried over by the! exhaust gases beingrecycled to the bed. Limestone of size lessl than l0 8.8.8. was added tothe fluidised bed.

Coal from Goldthorpe Colliery, England containing 2.1 per-' cent sulphurand 0.3 percent CO and coal from Ohio, U.S.A. containing 2.25 percentsulphur and 1.23 percent CO were burnt.

The results were as follows:

ZQ'This column gives the ratio of the weight of sulphur retainedl00/weight of sulphur which would have been retained stoichiometricallyaccording to the total CO fed in the bed in the coal and added carbonatecontaining substance.

in the test marked the ash bed has formed of merlite. In

5 the remaining tests of Ohio coal the ash bed was formed of ash 25ifrom coal from Babbington Colliery, England. In the tests withiGoldthorpe coal the ash was derived from Goldthorpe coal. In each testthe carbon content of the ash was less than 0.5 perl cent by weight. Theresults show, for example, that the addition of 12 percent limestone toGoldthorpe coal in the fluidised bed resulted in the emission of only 2percent of the original sulphur content of the coal in the exhaustgases. In this test the exhaust gases contained only 35 p.p.m. of S0 andAbout percent of the retained sulphur was retained in the ash and 10percent in the secondary fines and dust in the exhaust gas leaving asecondary cyclone separator.

The carbonate inherent in the Ohio coal led to the retention of aquarter of its sulphur content in the ash without the addition oflimestone or other carbonate. On the addition of limestone so that abouttwice the stoichiometric quantity i.e.,

according to the equations CaCO CaO CO; and 209.0 +80 +O 2CaSO waspresent, only 19 percent of the sulphur was present in the exhaust gas.

The amount of carbonate added is preferably greater than thestoichiometric amount in order to retain the sulphur The sulphur contentof the exhaust gas remains constant over a continuous extended period ofcombustion even though the quantity of carbonate may buildup in thesystem.

We claim:

1. A process for the combustion of a carbonaceous material containingsulphur comprising the steps of feeding the material to a fluidised bedof ash containing no more than 0.5 percent by weight of carbon, burningthe carbonaceous material within said bed at a temperature in the range700C.-C., while feeding a carbonate such as calcium carbonate to the bedin the proportion of 2- 14 percent by weight of the weight :of saidmaterial, whereby sulphur from said material reacts' i with saidcarbonate and is retained by the ash. i 2. A process according to claim1 wherein the carbonate is lfinely divided limestone.

3. A process according to claim 2 wherein the amount of added limestoneis about twice the stoichiometric equivalent of the sulphur content ofthe material.

